[petsc-users] Starting point for Stokes fieldsplit

[Max Rudolph]

I am solving a 2D Stokes flow problem using a finite volume discretization, velocity (dof 0,1)-pressure (dof 2) formulation. Until now I have mostly used MUMPS. I am trying to use the PCFieldSplit interface now, with the goal of trying out the multigrid functionality provided through ML. I looked at these (http://www.mcs.anl.gov/petsc/documentation/tutorials/Speedup10.pdf) talk slides for some guidance on command line options and tried. When assembling the linear system, I obtain the global degree-of-freedom indices using DMDAGetGlobalIndices. The solver does not appear to be converging and I was wondering if someone could explain to me why this might be happening. Thanks for your help.

Max

petscmpiexec -n $1 $2 $3 \
-stokes_pc_fieldsplit_0_fields 0,1 -stokes_pc_fieldsplit_1_fields 2 \
-stokes_pc_type fieldsplit -stokes_pc_fieldsplit_type additive \
-stokes_fieldsplit_0_pc_type ml -stokes_fieldsplit_0_ksp_type preonly \
-stokes_fieldsplit_1_pc_type jacobi -stokes_fieldsplit_1_ksp_type preonly \
-stokes_ksp_view \
-stokes_ksp_monitor_true_residual \

  Residual norms for stokes_ solve.
  0 KSP preconditioned resid norm 2.156200561011e-07 true resid norm 1.165111661413e+06 ||r(i)||/||b|| 1.000000000000e+00
  1 KSP preconditioned resid norm 2.156200561011e-07 true resid norm 1.165111661413e+06 ||r(i)||/||b|| 1.000000000000e+00
  2 KSP preconditioned resid norm 5.176186579848e-08 true resid norm 5.747484453195e+06 ||r(i)||/||b|| 4.932990239085e+00
  3 KSP preconditioned resid norm 5.054067588022e-08 true resid norm 5.763708451168e+06 ||r(i)||/||b|| 4.946915082953e+00
  4 KSP preconditioned resid norm 3.556841873413e-08 true resid norm 4.649778784249e+06 ||r(i)||/||b|| 3.990843915003e+00
  5 KSP preconditioned resid norm 3.177972840516e-08 true resid norm 4.677248322326e+06 ||r(i)||/||b|| 4.014420657890e+00
  6 KSP preconditioned resid norm 3.100188857346e-08 true resid norm 4.857618195959e+06 ||r(i)||/||b|| 4.169229745814e+00
  7 KSP preconditioned resid norm 3.045495907075e-08 true resid norm 5.030091724356e+06 ||r(i)||/||b|| 4.317261504580e+00
  8 KSP preconditioned resid norm 2.993896937859e-08 true resid norm 5.213745290794e+06 ||r(i)||/||b|| 4.474888942808e+00
  9 KSP preconditioned resid norm 2.944838631679e-08 true resid norm 5.403745734510e+06 ||r(i)||/||b|| 4.637963822245e+00
 10 KSP preconditioned resid norm 2.898115557667e-08 true resid norm 5.596205405394e+06 ||r(i)||/||b|| 4.803149423985e+00
 11 KSP preconditioned resid norm 2.853548106431e-08 true resid norm 5.788329048801e+06 ||r(i)||/||b|| 4.968046617765e+00
 12 KSP preconditioned resid norm 2.810975464206e-08 true resid norm 5.978285004145e+06 ||r(i)||/||b|| 5.131083313418e+00
 13 KSP preconditioned resid norm 2.770253127208e-08 true resid norm 6.164551408716e+06 ||r(i)||/||b|| 5.290953316217e+00
 14 KSP preconditioned resid norm 2.731250834421e-08 true resid norm 6.346469882864e+06 ||r(i)||/||b|| 5.447091547575e+00
 15 KSP preconditioned resid norm 2.693850812065e-08 true resid norm 6.523343466838e+06 ||r(i)||/||b|| 5.598899816114e+00